Hydraulic element and displacement pump having such an element

ABSTRACT

The present invention concerns a hydraulic element comprising a sleeve-shaped casing and a piston reciprocable in a longitudinal direction in the sleeve-shaped casing. To provide a hydraulic element for use in displacement pumps for conveying small amounts of liquid, in which the danger of breakage or deflection of the hydraulic element is reduced, it is proposed according to the invention that the piston has a cavity extending in the longitudinal direction and having a cavity entry, wherein there is provided a pin which extends through the cavity entry into the cavity and which is connected to the sleeve-shaped casing at the sleeve-shaped casing or by way of an intermediate element so that in a reciprocating movement of the piston a relative movement occurs between the piston and the pin. The present invention further concerns a displacement pump having the hydraulic element according to the invention.

The present invention concerns a hydraulic element comprising a sleeve-shaped casing and a piston reciprocable in a longitudinal direction in the sleeve-shaped casing.

In addition the present invention concerns a displacement pump for conveying a liquid comprising a delivery chamber, a displacement element preferably in the form of a diaphragm, wherein the displacement element is reciprocable between a pressure position and a suction position, wherein a volume of the delivery chamber is smaller in the pressure position of the displacement element than a volume of the delivery chamber in the suction position of the displacement element, a hydraulic chamber, wherein the displacement element separates the delivery chamber from the hydraulic chamber, a hydraulic element, wherein the hydraulic element is connected to the hydraulic chamber, wherein the hydraulic element for applying a force to the displacement element has a hydraulic liquid arranged in the hydraulic chamber, wherein at least a part of the hydraulic element is reciprocable between a first and a second hydraulic position, and a drive for driving the hydraulic element.

To convey liquids from a reservoir into a container or a conduit various forms of pump are known from the state of the art. A form of pump which is frequently used is the displacement pump.

In the case of displacement pumps for conveying a liquid in most cases the volume of a delivery chamber is alternately increased and reduced in size. During the increase in the volume a reduced pressure is produced in the delivery chamber whereby liquid is sucked into the delivery chamber by way of a suction connection. When subsequently the volume of the delivery chamber is reduced again the liquid which has been sucked in is urged out of the delivery chamber of the pump by way of a pressure connection.

To alter the volume of a delivery chamber a displacement pump has a displacement element reciprocable between a suction position and a pressure position. The movement of the displacement element can be produced in various ways. The displacement element can be reciprocated for example either by a mechanism fixed directly to the displacement element, or the displacement element can be reciprocated by a variation in pressure in an intermediate medium, for example a hydraulic liquid which exerts a force on the displacement element. In that case the pressure in the hydraulic chamber is typically varied by the movement of a moveable piston of a hydraulic element. The hydraulic liquid leads to force coupling between the hydraulic element and the displacement element.

The volume of the amount of liquid conveyed depends directly on the dimension or the effective area and the movement distance of the displacement element, by which the delivery chamber is alternately increased or reduced. In the case of an additional hydraulic means for producing the movement of the displacement element the volume of the liquid conveyed therefore also depends indirectly on the dimension or the effective area and the movement distance of the hydraulic element which causes a variation in pressure in the hydraulic chamber.

If very small amounts of liquid are to be conveyed then the variation in volume of the delivery chamber and thus possibly also the variation in volume of the hydraulic chamber must be correspondingly slight. That in turn requires correspondingly small displacement elements in the delivery chamber or hydraulic elements in the hydraulic chamber.

The disadvantage of those very small hydraulic and displacement elements is that they are difficult to handle as the pistons can very rapidly break when for example an element in the displacement pump has to be changed. The wear of such delicate elements is also higher so that the service lives of corresponding displacement pumps are reduced in comparison with those which have larger hydraulic and/or displacement elements. The production of such elements is also made more difficult as they easily bend.

In addition a special cost-intensive drive mechanism is necessary for the drive for such hydraulic and displacement elements as small hydraulic or displacement elements cannot carry the forces occurring in commercially usual drives without being ruined.

Therefore the object of the present invention is to provide a hydraulic element or a displacement element for use in displacement pumps for conveying small amounts of liquid, in which the risk of breakage or deflection of the hydraulic or displacement element is reduced.

According to the invention that object is attained by a hydraulic element comprising a sleeve-shaped casing and a piston reciprocable in a longitudinal direction in the sleeve-shaped casing, wherein the piston has a cavity extending in the longitudinal direction and having a cavity entry, wherein there is provided a pin which extends through the cavity entry into the cavity and which is connected to the sleeve-shaped casing at the sleeve-shaped casing or by way of an intermediate element so that in a reciprocating movement of the piston a relative movement occurs between the piston and the pin.

The hydraulic element according to the invention can be used to exert a force on a hydraulic liquid which is disposed in the hydraulic chamber. So that the hydraulic element can exert a force the hydraulic element typically comprises two components, namely a sleeve-shaped casing and a piston guided in the longitudinal direction therein. A movement of the piston then has the result that more or less force is exerted on a hydraulic liquid. In that case the force is decisively determined by the product of the cross-section of the piston and the movement distance, that is to say a stroke, of the piston in the longitudinal direction.

According to the invention it is provided that the cross-sectional area of the piston is reduced by a cross-sectional area of a pin which extends in a cavity in the piston. The remaining cross-sectional area of the piston, after deduction of the cross-sectional area of the pin, is referred to as the effective piston surface in accordance with the invention.

The term cross-sectional area of the piston in the present invention, in contrast to an effective piston surface, is used to denote the surface which extends perpendicularly to the longitudinal direction of the piston and is within the outside edges of the piston. In hydraulic elements known from the state of the art the total cross-sectional area of the piston corresponds to the effective piston surface.

The reduction according to the invention in the effective piston surface by the introduction of a pin in a cavity in the piston affords the advantage that the effective piston surface which is crucial for the change in volume, that is to say the cross-sectional area of the piston less the cross-sectional area of the pin, and thus the amount of liquid which can be conveyed, can be reduced without anything having to be changed at the outside dimensions of the piston or the inside dimensions of the sleeve-shaped casing. It is therefore possible to adopt a piston of such outside dimensions in respect of which, having regard to the forces which usually act, neither breakage nor deflection of the piston is to be expected, but at the same time a smaller amount of liquid can be conveyed than in the case of a piston with the same outside dimensions but without a cavity and a pin extending therein.

For that purpose the piston and the pin are so arranged relative to each other that the pin is at least partially disposed in the cavity in the piston and a relative movement is effected between the piston and the pin by the movement of the piston. In that case it is primarily the piston that moves and the pin remains in its position.

To achieve the relative movement between the piston and the pin the pin is either directly fixed to the sleeve-shaped casing of the hydraulic element or the pin is fixed to an intermediate element which in turn is itself connected to the sleeve-shaped casing. In any event preferably a fixed position of the pin in the hydraulic element is achieved in that way.

There are various possible ways available for fixing the pin to the sleeve-shaped casing or an intermediate element. In an embodiment the pin at its end towards the piston has corresponding contact surfaces which can be brought into engagement with corresponding contact surfaces of the sleeve-shaped casing or the intermediate element. In that case it is possible to envisage using both screw connections and also positively locking connections like a bayonet connection.

In an embodiment the piston of the hydraulic element is cylindrical and the sleeve-shaped casing is hollow-cylindrical, wherein an outside diameter of the piston substantially corresponds to an inside diameter of the sleeve-shaped casing. Accordingly the piston almost completely fills the sleeve-shaped casing at least in the radial direction. It will be appreciated that there must be a certain difference between the inside diameter of the sleeve-shaped casing and the outside diameter of the piston to permit movement of the piston in the sleeve-shaped casing. In a preferred embodiment however that is so small that only a negligible amount of hydraulic fluid can pass in operation through the gap between the outside surface of the piston and the inside surface of the casing.

It is additionally possible to provide a sealing element in the gap to prevent or at least reduce the flow of hydraulic liquid therethrough.

Such a configuration of the hydraulic element affords the advantage that both the sleeve-shaped casing and also the piston are easy to manufacture.

In a further embodiment the hydraulic element is so designed that the cavity in the longitudinal direction and/or the pin is or are of a circular cross-section. That affords the advantage that on the one hand the cavity can be produced for example by simple drilling or on the other hand the pin can be selected from easily available but highly precise standard components which do not have to be produced specifically for the hydraulic element according to the invention but can be inexpensively acquired in large amounts.

In a further embodiment of the hydraulic element according to the invention the cavity in the longitudinal direction, at least in the portion in which the pin is arranged almost completely in the cavity at a moment in time of the relative movement, is of a constant cross-section and the pin in a cross-section substantially completely fills the cavity so that the effective piston surface is substantially formed by the cross-sectional area of the piston less the cross-sectional area of the cavity.

In the case of a circular cross-section for the cavity and the pin an inside diameter of the cavity thus substantially corresponds to an outside diameter of the pin so that the pin substantially completely fills the cavity at least in the radial direction.

Here too it will be appreciated that there must be a certain difference between the cross-sectional area of the cavity and the cross-sectional area of the pin so that the cavity can be moved in relation to the outside surface of the pin. Nonetheless the cavity of the piston is sealed by the pin in such a way that almost no further substances like for example the hydraulic liquid or impurities can penetrate into the cavity by way of the outside surface of the pin.

The object of the invention is further attained by a displacement pump for conveying a liquid comprising a delivery chamber, a displacement element preferably in the form of a diaphragm, wherein the displacement element is reciprocable between a pressure position and a suction position, wherein a volume of the delivery chamber is smaller in the pressure position of the displacement element than a volume of the delivery chamber in the suction position of the displacement element, a hydraulic chamber, wherein the displacement element separates the delivery chamber from the hydraulic chamber, a hydraulic element, wherein the hydraulic element is connected to the hydraulic chamber, wherein the hydraulic element for applying a force to the displacement element has a hydraulic liquid arranged in the hydraulic chamber, wherein a part of the hydraulic element is reciprocable between a first and a second hydraulic position, and a drive for driving the hydraulic element, wherein the hydraulic element is a hydraulic element as described hereinbefore.

As described in the opening part of this specification the displacement element can be moved by a mechanical means fixed directly to the displacement element or, as claimed in an embodiment of the invention, it can be moved indirectly by an interposed hydraulic means. In that case the displacement element is preferably in the form of a diaphragm. Besides the displacement element the displacement pump according to the invention in this embodiment additionally has a hydraulic element, wherein the hydraulic element produces the movement of the displacement element by force coupling between the hydraulic element and the displacement element.

The force coupling between the displacement element and the hydraulic element is effected by way of a hydraulic liquid arranged in a hydraulic chamber. The volume of the hydraulic chamber and thus the pressure in the hydraulic chamber, which acts on the diaphragm, that is to say the displacement element, is alternately increased and reduced by the movement of the piston of the hydraulic element. As a result a volume of the delivery chamber is also alternately increased and reduced so that the liquid is alternately sucked in and discharged from the displacement pump.

The volume of the liquid conveyed depends on the change in pressure or volume in the hydraulic chamber, which occurs by virtue of the movement of the piston in the sleeve-shaped casing of the hydraulic element. The variation in pressure or volume in the hydraulic chamber in turn depends primarily on the product of the effective piston surface and the stroke of the piston, as already described hereinbefore.

By the piston having a cavity, into the cavity entry of which a pin extends, the effective piston surface is reduced by the cross-section of the pin. Therefore the term effective piston surface is used in accordance with the patent to denote the surface which still remains of the cross-sectional area of the piston after deduction of the cross-sectional area of the pin. Thus the piston according to the invention, in spite of being of comparatively large outside dimensions, has a small effective piston surface for conveying small amounts of liquid.

Preferably the cross-section of the pin is so adapted to the cross-section of the cavity that the pin substantially completely fills the cavity in a cross-section, wherein preferably the cavity in the longitudinal direction, at least in the portion in which the pin is arranged in the pressure position, is of a constant cross-section. The volume of the liquid which can be conveyed with the displacement pump according to the invention is thus substantially determined by the extent of the cavity in a direction perpendicular to the longitudinal direction of the sleeve-shaped casing or piston.

That affords the advantage that, for the displacement pump according to the invention, it is possible to use pistons which are easier to manufacture from the production engineering point of view than very thin pistons of correspondingly small cross-sectional area. Pistons of larger dimensions break less easily and also warp less severely during manufacture.

In addition the displacement pump according to the invention affords the advantage that the inside diameter or inside shape of the sleeve-shaped casing in itself does not have to be changed to convey a different volume of a liquid. To change the volume which can be conveyed by the displacement pump according to the invention it is provided that only the cavity in the piston and the pin extending in the cavity are adapted in their shape in order in that way to alter the effective piston surface and thus the volume of the liquid being conveyed. That affords the advantage that the displacement pump according to the invention can be easily adapted to different amount to be conveyed without expensive components of the displacement pump like for example the sleeve-shaped casing which is frequently cast in one piece with other pump components having to be replaced.

In addition the hydraulic element according to the invention in the displacement pump according to the invention can be operated with a conventional drive as the piston of the hydraulic element is sufficiently stable to carry the forces of a conventional drive. The drive that can be considered can be for example a linear motor which substantially comprises a stationary element and an element which moves relative to the stationary element.

In an embodiment of the displacement pump the piston is subjected to the action of hydraulic liquid only on the side towards the pin and preferably the drive for moving the piston has a crankshaft and a connecting rod, wherein the connecting rod is connected to the piston and the crankshaft in such a way that a rotary movement of the crankshaft is converted into an oscillating linear movement of the piston by the connecting rod. Both linear motors and also rotary motors are sufficiently known from the state of the art and can be inexpensively incorporated into a displacement pump according to the invention. In that respect the motors do not have to fulfil any particular demands in regard to the transmission of force as the piston to be moved enjoys adequate stability.

In a further embodiment of the invention the ratio of the cross-sectional area of the piston to the cross-sectional area of a portion of the pin, in which the pin extends into the cavity, is at most three and preferably at most two. In that way amounts of liquid which are as small as possible can be conveyed in spite of comparatively large piston dimensions. With the claimed relationships the arrangement also ensures that the piston still has sufficiently high stability, that is to say the piston neither breaks nor deflects under the forces which usually act.

In a further embodiment at its end remote from the cavity entry the cavity has a cavity outlet, through which a fluid can leave the cavity, wherein the cavity outlet is preferably connected to a reservoir. That is advantageous as fluid can escape or flow in through the cavity outlet when the piston moves.

The object of the invention is also attained by a displacement pump for conveying a liquid comprising a delivery chamber and a displacement element, wherein a part of the displacement element is reciprocable between a pressure position and a suction position, wherein a volume of the delivery chamber is smaller in the pressure position of the displacement element than a volume of the delivery chamber in the suction position of the displacement element and wherein the displacement element is a hydraulic element according to the foregoing description and wherein the pin is arranged in the delivery chamber.

In displacement pumps known from the state of the art the entire displacement element is usually moved. As in the case of the present invention however the displacement element is replaced by a hydraulic element according to the invention which is composed of non-moving components and a moving component, this embodiment of the invention provides that only a part of the displacement element is moved.

In this embodiment of the displacement pump according to the invention therefore the liquid is conveyed directly by the hydraulic element and not indirectly by a force which acts on the displacement element and which is generated by the hydraulic element in the hydraulic chamber.

This embodiment is particularly suitable for conveying liquids in which contact with the hydraulic element is not an issue. They include in particular non-etching substances which do not lead to degradation of the surfaces of the hydraulic element. In addition this embodiment of the displacement pump affords the same advantages for conveying small amounts of liquid as the above-described embodiments.

The hydraulic element according to the invention can also be used as a pneumatic cylinder.

Further advantages, features and possible uses of the present invention will be apparent from the description hereinafter of embodiments and the accompanying Figures. Identical components are denoted by the same references.

FIG. 1 shows a diagrammatic view of an embodiment of the displacement pump according to the invention,

FIG. 2 shows a perspective view of an embodiment of the hydraulic element according to the invention, and

FIG. 3 shows a diagrammatic view of a further embodiment of the displacement pump according to the invention.

FIG. 1 shows a displacement pump 1 for conveying a liquid comprising a delivery chamber 20, a displacement element 21 and a hydraulic element 10 driven by a drive (not shown). The hydraulic element 10 is connected to a hydraulic chamber 16 filled with a hydraulic liquid. The hydraulic liquid leads to a force coupling between the hydraulic element 10 and the displacement element 21. The hydraulic chamber 16 is separated from the delivery chamber by a diaphragm as the displacement element 21.

During operation of the displacement pump 1 a piston 12 of the hydraulic element 10 is reciprocated by the drive in a case-shaped sleeve 11. The force which acts on the displacement element 21 by virtue of the hydraulic liquid is altered by the movement of the piston 12.

If in FIG. 1 the piston 12 is moved towards the left into a first hydraulic position the force on the displacement element 21 is increased. That in turn leads to a movement of the displacement element 21 into a pressure position in which the volume of the delivery chamber 20 is smaller than a volume of the delivery chamber 20 in a suction position. If conversely in FIG. 1 the piston 12 is moved towards the right into a second hydraulic position the force on the displacement element 21 is reduced and the element moves into the suction position.

The drive of the hydraulic element 10 comprises a crankshaft and a connecting rod, the connecting rod being fixed directly to the piston 12 so that a rotary movement of the crankshaft is converted by the connecting rod into an oscillating linear movement of the piston 12.

During operation of the displacement pump 1 the drive reciprocates the piston 12 of the hydraulic element 10 alternately between the first and second hydraulic positions so that the displacement element 21 also reciprocates between a pressure and a suction position. In the suction position liquid is sucked into the delivery chamber 20 by way of a suction line 22. In the pressure position the sucked-in liquid is discharged from the delivery chamber 20 by way of a pressure line 23.

The volume of the liquid conveyed with the displacement pump 1 depends on the effective piston surface 18 of the piston 12 and the distance between the first and second hydraulic positions of the piston 12 in the longitudinal direction 40, wherein as shown in FIG. 2 the effective piston surface 18 is formed from the cross-sectional area of the piston 12 less the cross-sectional area 19 of the pin 15.

FIG. 2 shows the structure of the hydraulic element 10 according to the invention, fitted in the displacement pump 1. The hydraulic element 10 in itself has a sleeve-shaped casing 11 and a piston 12 reciprocable in a longitudinal direction in the sleeve-shaped casing. The casing 11 has a cylindrical cavity for guiding the piston 12. The piston 12 itself is cylindrical so that the inside diameter of the sleeve-shaped casing 11 substantially corresponds to the outside diameter of the piston 12.

The piston 12 has a cavity 13 extending in the longitudinal direction 40. At one end the cavity 13 has a cavity entry 14. A pin 15 extends through the cavity entry 14 into the cavity 13 of the piston 12. The cavity 13 of the piston 12 and the pin 15 are also cylindrical so that the pin 15 substantially completely fills the cavity 13 in the radial direction.

The outside diameter of the piston 12 is 16 mm and the outside diameter of the pin 15 is 11 mm so that a maximum volume of 1.7 ml per stroke of the displacement element 21 can be conveyed. The relationship of the cross-sectional area of the piston to the cross-sectional area 19 of the pin 15 is thus about 1.45.

The pin 15 is fixed at the sleeve-shaped casing 11 of the hydraulic element 10 by a securing ring 30 (see FIG. 1) so that, when the piston 12 is moved by the drive, the piston 12 performs a relative movement with respect to the pin 15 and the pin 15 remains substantially in a fixed position.

In the suction position or second hydraulic position the pin 15 fills the cavity 13 of the piston 12 in the axial direction to a lesser degree in comparison with the pressure position shown in FIG. 1. The cavity 13 of the piston 12 is therefore filled to a greater degree with the pin 15 in the pressure position or first hydraulic position in the axial direction than in the suction position.

FIG. 3 shows another embodiment of the displacement pump 1′ according to the invention, wherein the hydraulic element 10 itself is used as the displacement element 21. Here the piston 12 is of just the same configuration as in the preceding embodiment. The drive for the piston 12 is effected by way of a linear motor which moves the piston 12 in a longitudinal direction 40 relative to the sleeve-shaped casing 11 and the pin 15 accommodated in the cavity 13.

In this embodiment a movement of the piston 12 leads directly to a change in the volume of the delivery chamber 20 whereby liquid is alternately sucked in and pushed out of the displacement pump 1′. In this case the pin 15 is arranged in the delivery chamber 20 and fixedly connected to the sleeve-shaped casing 11.

For the purposes of the original disclosure it is pointed out that all features as can be seen by a man skilled in the art from the present description, the drawings and the claims, even if they are described in specific terms only in connection with certain other features, can be combined both individually and also in any combinations with others of the features or groups of features disclosed here insofar as that has not been expressly excluded or technical aspects make such combinations impossible or meaningless. A comprehensive explicit representation of all conceivable combinations of features is dispensed with here only for the sake of brevity and readability of the description.

While the invention has been illustrated and described in detail in the drawings and the preceding description that illustration and description is only by way of example and is not deemed to be a limitation on the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

Modifications in the disclosed embodiments are apparent to the man skilled in the art from the drawings, the description and the accompanying claims. In the claims the word ‘have’ does not exclude other elements or steps and the indefinite article ‘a’ does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude the combination thereof. References in the claims are not deemed to be a limitation on the scope of protection.

LIST OF REFERENCES

-   1, 1′ displacement pump -   10 hydraulic element -   11 sleeve-shaped casing -   12 piston -   13 cavity -   14 cavity entry -   15 pin -   16 hydraulic chamber -   18 effective piston surface -   19 cross-sectional area of the pin -   20 delivery chamber -   21 displacement element -   22 suction line -   23 pressure line -   30 securing ring -   40 longitudinal direction 

1. A hydraulic element (10) comprising a sleeve-shaped casing (11) and a piston reciprocable in a longitudinal direction (40) in the sleeve-shaped casing (11), characterised in that the piston (12) has a cavity (13) extending in the longitudinal direction (40) and having a cavity entry (14), wherein there is provided a pin (15) which extends through the cavity entry (14) into the cavity (13) and which is connected to the sleeve-shaped casing (11) at the sleeve-shaped casing (11) or by way of an intermediate element so that in a reciprocating movement of the piston (12) a relative movement occurs between the piston (12) and the pin (15).
 2. A hydraulic element (10) according to claim 1 characterised in that the piston (12) is cylindrical and the sleeve-shaped casing (11) is hollow-cylindrical, wherein the outside diameter of the piston (12) substantially corresponds to the inside diameter of the sleeve-shaped casing (11).
 3. A hydraulic element (10) according to claim 1 characterised in that the cavity (13) in the longitudinal direction (40) and/or the pin (15) is of a circular cross-section.
 4. A hydraulic element (10) according to claim 1 characterised in that the cavity (13) is of a constant cross-section in the longitudinal direction (40) and the pin (15) in a cross-section substantially completely fills the cavity (13) so that the effective piston surface (18) is substantially formed by the cross-sectional area of the piston (12) less the cross-sectional area (19) of the cavity (13).
 5. A displacement pump (1) for conveying a liquid comprising a delivery chamber (20), a displacement element (21), wherein the displacement element (21) is reciprocable between a pressure position and a suction position, wherein a volume of the delivery chamber (20) is smaller in the pressure position of the displacement element (21) than a volume of the delivery chamber (20) in the suction position of the displacement element (21), a hydraulic chamber (16), wherein the displacement element (21) separates the delivery chamber (20) from the hydraulic chamber (16), a hydraulic element (10), wherein the hydraulic element (10) is connected to the hydraulic chamber (16), wherein the hydraulic element (10) for applying a force to the displacement element (21) has a hydraulic liquid arranged in the hydraulic chamber (16), wherein a part of the hydraulic element (10) is reciprocable between a first and a second hydraulic position, and a drive for driving the hydraulic element (10), characterised in that the hydraulic element (10) is a hydraulic element (10) according to claim
 1. 6. A displacement pump (1) according to claim 5 characterised in that the ratio of the cross-sectional area of the piston (12) to the cross-sectional area (19) of a portion of the pin (15), in which the pin (15) extends into the cavity (13), is not more than
 3. 7. A displacement pump (1) according to claim 5 characterised in that the piston (12) is subjected to the action of hydraulic liquid only on the side towards the pin (15), wherein the connecting rod is connected to the piston (12) and the crankshaft in such a way that a rotary movement of the crankshaft is converted into an oscillating linear movement of the piston (12) by the connecting rod.
 8. A displacement pump (1) according to claim 5 characterised in that at its end remote from the cavity entry (14) the cavity (13) has a cavity outlet, through which a fluid can leave the cavity, wherein a fluid can flow into the cavity from the reservoir through the cavity outlet.
 9. A displacement pump (1′) for conveying a liquid comprising a delivery chamber (20), a displacement element (10), wherein a part of the displacement element (10) is reciprocable between a pressure position and a suction position, wherein a volume of the delivery chamber (20) is smaller in the pressure position of the displacement element (10) than a volume of the delivery chamber (20) in the suction position of the displacement element (10), characterised in that the displacement element (10) is a hydraulic element (10) according to claim 1, wherein the pin (15) is arranged in the delivery chamber (20).
 10. A displacement pump (1) for conveying a liquid according to claim 5 wherein the displacement element (21) is in the form of a diaphragm.
 11. A displacement pump (1) according to claim 5 wherein the ratio of the cross-sectional area of the piston (12) to the cross-sectional area (19) of a portion of the pin (15), in which the pin (15) extends into the cavity (13), is not more than
 2. 12. A displacement pump (1) according to claim 7 wherein the drive has a crankshaft and a connecting rod.
 13. A displacement pump (1) according to claim 8 wherein the cavity outlet is connected to a reservoir. 